The present invention relates to a direct injection gasoline engine,
In direct injection gasoline engines, a combustion chamber is delimited in each cylinder by a longitudinally movable piston and by the inner wall of a cylinder head, an injector injecting fuel into the combustion chamber in order internally to form a fuel/air mixture with combustion air that is supplied separately. The composition of the fuel/air mixture must be within the ignitable window in the area of the spark plug in order to be ignitable by an ignition spark, which is triggered between the electrodes of a spark plug.
U.S. Pat. No. 5,577,473 describes an injection nozzle for direct introduction of fuel or fuel-air mixture in a combustion chamber of an internal combustion engine. The injection nozzle includes a valve needle which opens to the outside, and between its valve head and a cone-shaped valve seat an umbrella-like injection jet is formed from fuel or fuel-air mixture, the lateral surface of which points toward the spark plug. The quantity of fuel that is introduced is metered so that the internal combustion engine is operated with a lean fuel-air mixture, the air/fuel ratio (lambda) of which is on average  greater than 1, a ring-shaped guide surface area being provided next to the valve seat in the outlet direction of the injection jet, which has a recess that deviates from the ring shape in an area that is oriented towards the spark plug.
European Published Patent Application No. 0 835 994 describes a direct injection internal combustion engine, which includes a shed roof-shaped combustion chamber and an injector arranged in the center, the electrodes of the spark plug being arranged near the intake valve. The fuel is injected in the form of a hollow cone into a piston, which includes a cavity with a circular-shaped projecting part, and the fuel impinges on the piston recess. The fuel which thus becomes sprayed is transported by a tumble flow to the electrodes of the spark plug, the circular-shaped projecting part of the piston recess preventing the sprayed fuel from being scattered in the direction of the cylinder wall, whereby a stable stratified charge combustion is ensured.
German Published Patent Application No. 195 46 945 describes a direct injection internal combustion engine, the injectors of which inject the fuel into the combustion chamber in a cone shape via their injection nozzles, the spark plug being arranged in such a way that its electrodes are outside of the lateral surface of the fuel cone generated by the injection nozzle. This arrangement prevents wetting of the electrodes with fuel during the injection operation and counteracts soot deposition on the electrodes because of incompletely burned fuel. The electrodes are free of coking over a long operating time period, whereby an orderly operation of the internal combustion engine without misfiring should be ensured. In order to bring the ignitable mixture between the electrodes arranged outside of the fuel cone, the spark plug is to be arranged in such a way that the ground electrode is at a small distance from the lateral surface of the fuel cone and the inner wall of the cylinder head extends parallel to the lateral surface of the fuel cone while forming a gap at least where the electrodes of the spark plug are arranged.
In the gap, a turbulent flow should be produced, which is composed of a fuel/air mixture and which extends into the area of the electrodes. In order to generate the turbulent flow, a special shape of the inner wall and an arrangement of the spark plug near the injector is necessary. The injector is arranged in a recess of the inner wall, i.e., set back from the free combustion chamber volume, whereby the mixture vortex is produced in the area adjacent to the injection nozzle and should circulate in the hollow space that is formed between the lateral surface of the fuel cone and the inner wall of the cylinder head in the area of the injection nozzle. Furthermore, air, which was displaced by the fuel that was injected into the combustion chamber, should flow back through the air gap between the fuel cone and the parallel, likewise cone-shaped inner wall of the cylinder head. During the return flow to the spark plug along the inner wall, additional small portions of the fuel should also be entrained from the fuel cone. The turbulent flow is formed so that it is sufficiently pronounced in the area near the injector, in order to bring an ignitable mixture between the electrodes of a spark plug. The spark plug must therefore be arranged near the injector.
In the conventional direct injection gasoline engines, the combustion chamber perimeter must be precisely designed at a high expense, especially through the inner wall of the cylinder head, in order to obtain the desired hydromechanical effects to form the ignitable mixture vortex. The conventional combustion chamber configuration with the combustion chamber shape necessary to form the mixture vortex and the spark plug necessarily arranged near the injector often cannot achieve optimum combustion or ensure the desired operating performance of the internal combustion engine.
It is an object of the present invention to provide a direct injection gasoline engine so that the internal combustion engine operates with optimum operating performance.
In the combustion chamber configuration according to the present invention, the fuel cone is injected in a free jet that is substantially unaffected by the combustion chamber perimeter, i.e., the fuel cone is injected at a sufficiently large distance, in particular, from the inner wall of the cylinder head, that the cone-shaped fuel jet spreads out in the free combustion chamber volume substantially without hydromechanical wall effects of the combustion chamber perimeter. In the process, vortices of fuel emerging from the lateral surface of the cone form during the injection. These vortices are composed at first mainly of fuel vapor and mix with the surrounding combustion air in the combustion chamber. The fuel vortices form in a particularly pronounced manner if the cone angle of the fuel jet cone is between 70xc2x0 and 110xc2x0 and they are generated by an air flow that occurs in the area of the lateral surface of the fuel cone because of air entrained by the fuel jet, while an air flow is also generated in the opposite direction by the resulting vacuum. The spark plug is positioned according to the present invention so that the electrodes project into the fuel vortex of the free jet. For example, the spark position of the electrodes is 1 mm to 15 mm away from the lateral surface of the fuel cone.
The fuel vortex, which brings an ignitable mixture between the electrodes, forms on the lateral surface of the free jet without effective influence of the combustion chamber perimeter, so that the combustion chamber shape may be configured freely. A jet-guided combustion process is present in which wall effects of the inner wall of the cylinder head or possibly a piston cavity barely exert an influence on the mixture formation or the ignition. In particular, in the stratified charge operation of an internal combustion engine, when fuel is injected during the compression stroke and a central fuel cloud is formed when the combustion chamber is filled with air, an optimal burn-through of the combustion chamber charge may be achieved with a simple combustion chamber configuration. Another advantage of the mixture formation according to the present invention is that the spark plug may be arranged further away from the injector. The fuel vortex remains stable for a long time almost at the same position in the combustion chamber, so that ignition may occur independently of the injection point over a wide time interval.
The free fuel jet is injected, for example, into the combustion chamber in a hollow cone shape. In this manner, the fuel vortices form in a shape that is particularly suitable for the transport of the mixture to the spark plug, in particular, for injection at a high cylinder pressure in the compression phase during the stratified charging operation. In order to form the hollow cone jet, an injector with an injection nozzle that opens to the outside may be used. The injection nozzle may in this case be constructed so that the fuel emerges from the injector as perpendicularly as possible to the surface of the opening valve element, so as to counteract depositions and coking. Injection nozzles with swirl generators may be used or even injectors with two magnetic coils for moving the valve element opening to the outside. Also, injection nozzles that open to the inside, i.e., into the inner space of the injector may be provided, which generate a distinctive hollow cone jet. In this manner, a higher fuel concentration is produced on the edge of the jet with more than ⅔ of the entire injected amount in the outer third of the fuel cone. In order to form the hollow cone jet, injectors with multi-hole nozzles may also be used, the fuel openings of the multi-hole nozzle being arranged so that a hollow cone jet is formed from the individual jets passing through during fuel injection. Fundamentally, any injector that generates a distinctive hollow cone jet with its constructive design may be suitable for the fuel injection in a free jet according to the present invention.
In order to form a distinctive fuel vortex on the injection cone, the injector is arranged so that an angle between an axis of symmetry of the fuel cone and a cylinder axis of the cylinder is less than 25xc2x0. The injection nozzle may be at a distance of less than 20 mm away from the cylinder axis. The injector may be arranged centrally in the combustion chamber, the axis of symmetry of the injected fuel cone coinciding with the axis of the cylinder.
In another example embodiment of the present invention, two spark plugs are provided per cylinder. Through a double ignition, in which both spark plugs form ignition sparks, the risk of misfiring may be reduced. Also, under extreme operating conditions of the internal combustion engine, if possibly the fuel vortex transports too lean a mixture between the electrodes of a spark plug, ignition may still be ensured through the other spark plug. The two spark plugs may be arranged at equal distance from the injector in the combustion chamber. If the spark plugs with their respective ignition positions are located at different distances from the injector, then depending on the operating point of the internal combustion engine, the spark plug that is used for ignition may be the one more favorably positioned with regard to the formation of the fuel vortex. The position of the fuel vortex is affected by the counterpressure in the combustion chamber, so that the optimum spark position for ignition is variable in the characteristics map of the internal combustion engine depending on the operating conditions of the internal combustion engine, such as the injection time. In this manner, the ignition of the fuel vortex may be ensured in any case by one of the igniting spark plugs having different spark positions relative to the fuel cone.
A control unit may be provided which determines, as a function of the operating conditions, which of the two spark plugs is used to ignite the fuel/air mixture. In the process, depending on the operating mode (stratified charge or homogeneous mixture formation) and the operating conditions, the ignition at the most favorable ignition location is ensured, where the fuel vortices emerging from the fuel cone cover the corresponding spark plug.
The mixture formation in the combustion chamber may be improved by suitable steering of the inflowing combustion air. For example, the combustion air may be brought into the combustion chamber in a tumble motion, where the combustion air rotates in an approximately circular movement in a plane of the cylinder axis. Effective ignition is ensured in the process through the spark plug, which is arranged in a rear section of the flow path of the combustion air in the combustion chamber. In the case of a tumble flow with combustion air inflowing at first approximately parallel to the combustion chamber roof, the spark plug may be arranged in the area of the air admission, i.e., adjacent to the intake valve, for example, between two intake valves in multi-valve engines. In the case of reverse tumble flow, the spark plug may be correspondingly arranged on the outlet side. Furthermore, the mixture formation of the gasoline engine according to the present invention, with fuel injection in a free jet, may be improved through a swirl-shaped charging movement in the combustion chamber. With a swirling flow of the combustion air around the cylinder axis, asymmetries and skeins of the injected fuel jet may be reduced during the mixture formation and thus the ignition conditions may be improved in the area of the vortices that emerge on the fuel cone. The swirl may be generated by appropriately shaped intake channels, so-called swirl or spiral channels, through the offset of the intake valves or the rotated radial valve cluster or, in multi-valve engines, by switching off the valve or via adjustable throttle elements in the intake section.
The optimal cone angle of the fuel cone in the angle range between 70xc2x0 and 110xc2x0 for the formation of powerful fuel vortices in the jet edge area is dependent on the combustion chamber shape, in particular, on the setting angle of the valve axes of the gas exchange valves. In the case of a combustion chamber roof angle of 180xc2x0, the optimal jet angle of the fuel cone is approximately 90xc2x0. The cone angle of the fuel cone may be reduced by approximately 1xc2x0 to 2xc2x0 when the roof angle decreases by approximately 10xc2x0. Good mixture configurations are achieved in an acceptance range of approximately 20xc2x0 above and below the theoretically optimum cone angle of the cone jet, i.e., in the angular range of approximately 70xc2x0 to 110xc2x0.
Ignition may occur after the end of the injection operation approximately 0.1 ms to 1.5 ms after the end of injection.